Nitrous oxide fluxes in estuarine environments: response to global change

Abstract

Nitrous oxide is a powerful, long-lived greenhouse gas, but we know little about the role of estuarine areas in the global N2 O budget. This review summarizes 56 studies of N2 O fluxes and associated biogeochemical controlling factors in estuarine open waters, salt marshes, mangroves, and intertidal sediments. The majority of in situ N2 O production occurs as a result of sediment denitrification, although the water column contributes N2 O through nitrification in suspended particles. The most important factors controlling N2 O fluxes seem to be dissolved inorganic nitrogen (DIN) and oxygen availability, which in turn are affected by tidal cycles, groundwater inputs, and macrophyte density. The heterogeneity of coastal environments leads to a high variability in observations, but on average estuarine open water, intertidal and vegetated environments are sites of a small positive N2 O flux to the atmosphere (range 0.15-0.91; median 0.31; Tg N2 O-N yr(-1) ). Global changes in macrophyte distribution and anthropogenic nitrogen loading are expected to increase N2 O emissions from estuaries. We estimate that a doubling of current median NO3 (-) concentrations would increase the global estuary water-air N2 O flux by about 0.45 Tg N2 O-N yr(-1) or about 190%. A loss of 50% of mangrove habitat, being converted to unvegetated intertidal area, would result in a net decrease in N2 O emissions of 0.002 Tg N2 O-N yr(-1) . In contrast, conversion of 50% of salt marsh to unvegetated area would result in a net increase of 0.001 Tg N2 O-N yr(-1) . Decreased oxygen concentrations may inhibit production of N2 O by nitrification; however, sediment denitrification and the associated ratio of N2 O:N2 is expected to increase.

Nitrous oxide and methane in two tropical estuaries in a peat-dominated region of northwestern Borneo

“…The reason is obviously the missing relationship between DIN concentrations and N 2 O emissions in the studied system. Although across estuaries increasing DIN is generally associated with increasing N 2 O (Murray et al, 2015), the relationship is not linear : Zhang et al (2010) found a polynomial relationship between DIN and N 2 O across estuaries in different geographic locations. The emission factor from Mosier et al (1998), in contrast, is based on a constant N 2 O / DIN ratio.…”

“…The world's oceans, including its coastal zones, are sources of N 2 O and CH 4 to the atmosphere and play a major role in the global budget of atmospheric N 2 O, but only a minor role in the global budget of atmospheric CH 4 (Ciais et al, 2013). Rivers and estuaries are considered hot spots for the production and emission of both N 2 O and CH 4 (Bange, 2006;Bastviken et al, 2011;Borges et al, 2015;Murray et al, 2015;.…”

High Resolution Measurements of Nitrous Oxide (N2O) in the Elbe Estuary

“…Moreover, eutrophication via N inputs also promotes hypoxia or anoxia (Howarth et al, 2011) which, in turn, may lead to additional N 2 O production (see e.g., Naqvi et al, 2010). In line with this, estuaries are known to release high amounts of N 2 O from nitrification and/or denitrification (e.g., Bange, 2006;Barnes and Upstill-Goddard, 2011;Murray et al, 2015).…”

Spatial variability of nitrous oxide in the Minho and Lima estuaries (Portugal)

“…However, considerable efforts have been made in the last few decades to better understand the nitrogen cycle, the dynamics of N 2 O production and the quantification of the respective emission from European estuaries. More recently Murray et al (2015) reviewed N 2 O global fluxes from estuarine environments and reported a variation of 0.17-0.95 Tg N 2 O-N yr -1 .…”